Many types of semiconductor devices are made using similar manufacturing procedures. A starting substrate, usually a thin wafer of silicon, is doped, masked, and etched through several process steps, the steps depending on the type of devices being manufactured. This process yields a number of die on each wafer produced. Each die on the wafer is given a brief test for functionality, and the nonfunctional die are mechanically marked or mapped in software. This brief test is only a gross measure of functionality, and does not insure that a die is completely functional or has specifications that would warrant its assembly in a package.
If the wafer has a yield of grossly functional die which indicates that a good quantity of die from the wafer are likely to be fully operative, the die are separated with a die saw, and the nonfunctional die are scrapped while the rest are individually encapsulated in plastic packages or mounted in ceramic packages with one die in each package. After the die are packaged they are rigorously tested. Components which turn out to be nonfunctional or which operate at questionable specifications are scrapped or devoted to special uses.
Packaging unusable die only to scrap them after testing is a waste of time and materials, and is therefore costly. Given the relatively low profit margins of commodity semiconductor components such as dynamic random access memories (DRAMs) and static random access memories (SRAMs), this practice would seem especially wasteful. However, no thorough and cost effective method of testing an unpackaged die is available which would prevent this unnecessary packaging of nonfunctional and marginally functional die.
The practice of packaging die only to find the component must be scrapped can especially adversely affect yields on multi-chip modules (MCMs). With MCMs, several unpackaged die are assembled into a single component, then the component is tested as a single functional unit. If a single die is nonfunctional or operates outside of acceptable specifications, the entire component fails and all die in the package are scrapped or an attempt is made to "re-work" the MCM. There is presently no cost-effective way to reclaim the functioning die. Statistically, the yields of MCMs decrease in proportion to the increasing number of die in each module. The highest density modules have the lowest yields due to their increased total silicon surface area.